[173] 1 of caustic soda in 10-15 of water, boiled with 8 of stearic acid till clear, cooled to 25° C. and diluted with 400-800 water, with constant stirring, till the white jelly of suitable consistence is obtained. Somewhat similar, but harder preparations may be made with waxes, or fatty acids still higher than stearic.
[174] 5 parts of shellac digested warm with 100 water and 3 of ammonia fort., or 1 of borax. If the solution is used as a “seasoning” for glazing, the waxy matter which separates on standing should be mixed by shaking before use. As a varnish, a stronger solution should be used and the wax skimmed off.
In dyeing blacks on other than vegetable tannages, however, chrome becomes of importance, as logwood is principally employed, though sometimes in conjunction with tannin, and often with addition of quercitron or fustic, to correct the bluish shade of the logwood-chrome or logwood-iron lake. It must not be overlooked in practice, that if ferrous salts are mixed with bichromate solutions, the latter are reduced, and the iron is oxidised to the ferric state.
In alumed leathers the fixing power of the original hide-fibre is much less affected than in vegetable tannages. Whatever may be the truth with regard to the latter, there is little doubt that physical influences are at least as important as chemical ones in the production of mineral tannages. The amount of the tanning agent absorbed is greatly influenced by the concentration of the solutions, and in ordinary alum tawing much of the alumina may again be removed by free washing. In this case, the sulphate of potash present takes no part in the operation, but the alumina salt is absorbed apparently as a normal salt. Alum or alumina sulphate alone is incapable of producing any satisfactory tannage without the assistance of common salt, the quantity absorbed being small, and the fibre becoming swollen by the action of the acid. In presence of salt the absorption is greater, and the swelling is prevented. The explanation of this is not to be found in the formation of aluminium chloride, for though this undoubtedly takes place, it has been shown that the action of aluminium chloride without salt is not more satisfactory than that of alum. It has long been known that salt prevents the swelling action of acids on skin, although it does not lessen the absorption of acid; and the fact is capable of explanation on modern osmotic theories (cp. [p. 89]). The skin so treated is found to be converted into leather, but if the salt be washed out, the acid is retained by the skin, which returns to the state of acid-swollen pelt. It is probable, therefore, that although the acid and alumina are absorbed in equivalent proportions to each other, they are really dissociated, and attached to different groups in the gelatine molecule, and that the effect of the salt is to allow the absorption of the acid without swelling, and, osmotically, to increase the dissociating power of the pelt. If, in place of a normal alumina salt, a basic salt is employed, such as may be obtained by partial neutralisation of the sulphuric acid with soda, satisfactory tannage may be accomplished without salt, a basic compound is absorbed, and the leather is much less affected by washing. In the analogous case of chrome tannage, this basic compound may be still further deprived of its residual acid, by washing the tanned skin with alkaline solutions, leaving a leather which is extremely resistant even to hot water; and a somewhat similar result may be obtained with alumina, though with more difficulty, as apparently a very small excess of alkali destroys the qualities of the leather. (Cp. [p. 187].)
The results on dyeing are almost what might have been foreseen. While ordinary alumed leather absorbs both acid and basic dyes readily, the basic chrome leather has practically lost its affinity for the latter. Both chrome and alumina leathers readily absorb vegetable tannins, thus supporting the view that the acid-fixing groups of the gelatine molecule are still unsaturated (tannins are capable of tanning pelt swollen with sulphuric acid and apparently of expelling the acid). In the case of chrome leather the effect of re-tanning with tannins is greatly to lessen its stretch, and if carried too far, to destroy its toughness, but it at once becomes capable of fixing basic dyestuffs. This property is frequently made use of in dyeing, but the effect on the leather must not be disregarded where softness and stretch are important, as in the case of glove-leathers. Polygenetic dyes are, of course, fixed on alum- or chrome-leathers by the alumina- or chrome-mordant, though apparently the bases are not present in the most favourable condition for fixing colours. Thus logwood extracted without alkali dyes tanned leather yellow, alumed leather violet-blue, and chrome leather blackish-violet, and some of the alizarine group dye very well on chrome as its resistance to hot water allows much higher temperatures to be used than with most other leathers. The tannin contained in dyewoods has the effect of lessening the stretch of chrome leathers.
Something should perhaps be said on the dyeing of oil and aldehyde leathers, but the subject has as yet been scarcely treated scientifically, and our practical knowledge of the subject is insufficient to justify theorising. (See, however, [p. 496].)
Defects in the colour of the finished leather are due to a variety of causes, but many are produced by want of cleanliness and system during the dyeing itself. The greatest care is needed in this respect, and in brush-dyeing a different brush should be used for each different colour, as it is impossible to thoroughly remove all traces of dye by the ordinary methods of cleansing.
Irregular and surface dyeing sometimes occurs owing to too rapid fixation of the colours; while in other cases the affinity of the dye is too small to allow of reasonable exhaustion of the bath. Addition of salts of weak acids, such as potassium hydrogen tartrate (tartar), or of those like sodium sulphate, which form hydric salts, lessen rapidity of dyeing with acid colours; while acids generally increase it, and it is also often increased by addition of common salt, which lessens the solubility of the dye. Weak acids, such as acetic or formic, or acid salts, such as sodium bisulphate, are generally to be preferred to sulphuric acid as an addition to the dye-bath; and if the latter is used, great care is desirable in its complete removal. There is no doubt that the rapid decay of leather bookbindings and upholstery is largely due to the careless use of sulphuric acid in “clearing” and dyeing the leather;[175] and even if it is fully removed, it has saturated all bases such as lime, which are naturally present in leathers in combination with weak acids, and which would otherwise act as some protection from the sulphuric acid evolved in burning coal gas.
[175] See Report of Committee of Society of Arts on Bookbinding Leathers, 1901.
“Bronzing,” the dichroic effect produced by light reflected from the surface of many colouring matters, complementary to that transmitted by them and reflected by the surface of the dyed material, is not peculiar to basic colours, but is generally more marked in them than in acid ones. Basic colours, from their great affinity for tannins, and consequent rapid dyeing, are apt to dye irregularly, and without sufficiently penetrating the leather, and if the soluble tannin is not wholly washed out of the skins previously to dyeing, it bleeds in the dye-bath, and precipitates insoluble tannin-lakes, which waste colour and adhere to the surface of the leather. The inconvenience of basic colours due to their too rapid fixation may sometimes be lessened by slight acidification of the dye-bath with a weak acid, such as acetic or lactic. The acid may be still further “weakened” if desired, by the addition of its neutral (sodium) salt. The precipitation of tannin-lakes in the bath may be prevented by previous fixation of the tannin with tartar emetic, titanium potassium oxalate or lactate, or some other suitable metallic salt.